[Updated at 2:10 p.m. ET]More than a decade after a terrorist attack brought down New York's twin towers, their under-construction replacement became the city's tallest building on Monday.

The placement of a column of the 100th floor brought the colossal new steel structure of One World Trade Center tower to a height of 1,271 feet - surpassing the frame of the Empire State Building, which is currently New York's tallest skyscraper, by 21 feet.

The One World Trade Center building will have three top-floor observation decks. Its first 90 floors will be designated for office space, and the following 10 floors will be reserved for air conditioning, heating, and electrical equipment.

Despite years of political infighting and real estate squabbling that delayed its construction, more than half of the building has now been rented, with a tenant list that includes Conde Nast publishing company and a Chinese real estate investment firm called Vantone Holdings.

In its shadow, twin reflecting pools are situated in the footprints of where the twin towers once stood.

A memorial at the site of one of the original World Trade Center buildings.

The names of the nearly 3,000 people killed in the 2001 attacks, as well as six people who died in the World Trade Center bombing in 1993, are emblazoned across bronze panels ringing the pools.

The steel beams were placed atop the structure on Monday, one day before the one-year anniversary of Osama bin Laden's death. The project is bringing a major milestone to the city and country, said Mike Mennella, the construction executive who was in charge of building the first towers and is helping to build the new structure.

"Today is a day where we can really look back and say the milestones we’ve surpassed and overcome are certainly more significant than the ones ahead of us," he said. "The building is in position now where we can see it coming from the top, and see it being finished off in a very, very significant way."

As the man who built the first towers, only to watch them fall during the terrorist attacks, Mennella knows just how important it is to be able to rebuild and start a new chapter in lower Manhattan.

"It's also big in the region," he said. "Seeing this building from all over the region - Long Island, New Jersey, New York - it's just a statement for the region that we’ve reached a real milestone."

Mennella said he came to the site on September 12, 2001, amid the devastation and always hoped he'd be able to be part of an effort to rebuild. He felt it wasn't just about rebuilding an icon of New York's skyline, but about helping heal and move forward.

"I think as people look at the building from afar and then realize the site that it sits on, the memorial it adjoins and what the site is about, what was lost here, and what is now being put back together - it's an exciting moment," he said.

This new structure will be more bricks to attack again. this is crazy. WE CAN SURELY LOOK UP AND KNOW THAT OUR REDEMPTION DRAWS NEAR. THIS IS MANS PRIDE IN STRONGER FORM. A SIGN OF THE TIMES POINTING US TO THE CLOSE OF THIS EARTHS HISTORY. JESUS IS SOON RETURNING AND THIS IS JUST A SIGN OF HIS SOON RETURN.

I worked in two wtc on 9/11 and think that this building is a joke. The building is nothing more than an attempt at pataki and silverstein being able to justify the insurance and stimulus money they scanned out if the tragedy. Physically its a ghost of the original choice design. The setback and pedistal make it look small and uninspiring. Instead of pandering to the families they should have rebuilt the original towers taller and safer right in the footprints of the original. Don't even get me started on the billions for the subway upgrade.

Amazing...but what about people who are totally afraid of heights? I could never take a job in a building that tall unless they had office space on the third floor. I have worked on the 18th floor of a building once...they had to give me an office way away from the windows.

"The steel beams will be placed atop the structure on Monday, one day before the one-year anniversary of Osama bin Laden's death. "

What steel beams? This paragraph starts like it is in the middle of a thought. Did the author mean the final steel beams–like for the radio towers? Did anyone proofread this article? I swear, it freaks me out how many news articles I read each week from "professional" sites that have typos and errors in the text. Worse that they say so little and ignore obvious and important gaps in their "journalism."

It is simply beautiful to see the commitment and energy behind this project. It is obviously a sign of hope, perseverance, freedom and more importantly the foundation of the principals which this country was build on.

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